Problem: Which of the following numbers is a factor of 168? ${3,5,9,10,13}$
Solution: By definition, a factor of a number will divide evenly into that number. We can start by dividing $168$ by each of our answer choices. $168 \div 3 = 56$ $168 \div 5 = 33\text{ R }3$ $168 \div 9 = 18\text{ R }6$ $168 \div 10 = 16\text{ R }8$ $168 \div 13 = 12\text{ R }12$ The only answer choice that divides into $168$ with no remainder is $3$ $ 56$ $3$ $168$ We can check our answer by looking at the prime factorization of both numbers. Notice that the prime factors of $3$ are contained within the prime factors of $168$ $168 = 2\times2\times2\times3\times7 3 = 3$ Therefore the only factor of $168$ out of our choices is $3$. We can say that $168$ is divisible by $3$.